Field experiment report (January 2018)

This page presents results of a field experiment that was conducted at
Université Bretagne Sud (France)
in January 2018. The objective of this experiment was to observe how
opportunistic communication can be used to support messaging between
mobile users.

Mobility and application scenario

10 volunteers were equipped with HTC Wildfire-S smartphones, whose
Wi-Fi interfaces were configured to operate in ad hoc mode. Each
smartphone
ran DodwanDroid,
an Android application based
on DoDWAN, as well
as another application recording GPS locations and radio contacts.

During the experiment the volunteers were asked to roam a small
university
campus (covering roughly a 420 m x 160 m area), staying outdoors
(because of the GPS recording), while using the DodwanDroid app to
exchange short-text messages on a public channel (meaning every
message published by one volunteer was meant to be received by all
other volunteers). The experiment lasted about 25 minutes, but the
volunteers were requested to start DodwanDroid on their smartphone
shortly after they began walking in the campus, and to stop it shortly
before the 25-minute deadline. They were also requested to stop
publishing messages a couple of minutes before stopping the
DodwanDroid application, thus giving the last messages published a
chance to disseminate for a while.

Video of the field experiment

Log files were collected after the field experiment, and a video
was produced (among other things) based on data extracted from these
log files. This video shows how the volunteers moved during the field
experiment, and how their smartphones managed to exchange
messages. The video covers the whole duration of the field experiment
(i.e., a bit more than 25 minutes), but it is accelerated 4 times.

Each moving node in the video represents a volunteer, or more
specifically the smartphone he/she is carrying. A node is shown grey
as long as the DodwanDroid app is not running on the smartphone, and
it turns red when DodwanDroid is started.

An edge between two nodes blinks grey whenever a control message is
transferred between these nodes (in any direction). It blinks red for
the transfer of an application-level message (i.e., one of the
messages published by the volunteers).

This video has been produced using the LEPTON
platform. This platform is mostly meant to emulate an opportunistic
network, but it can also replay mobility traces, contact traces, and
application-level traces collected during field experiments such as
this one.

Analysis of log files

The log files collected after the field experiment have been
analysed. Some of the main statistical figures thus obtained are
shown in the table below (Table I).

Metric

Values (* = min / max / avg / stdev values)

Duration of the experiment

25’15"

Nb. of nodes involved in the experiment

10

Nb. of active nodes

2.0 / 10.0 / 9.5 / 1.5 (*)

Activity duration per node

21’35" / 23’53" / 23’06" / 41" (*)

Average number of neighbors per node

0.0 / 2.7 / 1.2 / 0.5 (*)

Number of contacts

163

Durations of contacts

00" / 10’16" / 48" / 01’11" (*)

Number of inter-contacts

119

Durations of inter-contacts

04" / 18’12" / 04’01" / 03’37" (*)

Number of messages published

249

Number of receive events

2234 (delivery ratio: 99.6%)

Message delivery delays

00" / 18’16" / 02’30" / 02’08" (*)

Distances covered during msg transfers (m.)

0.0 / 171.8 / 55.7 / 32.6 (*)

Table I

Transmission distances

In the analysis below any message considered is an
application-level message, that is, a message published by one
of the volunteers during the field experiment. The transmission of
such a message is depicted by an edge blinking red in the video
presented above.

The last line in Table I provides interesting figures about the
distances covered by these messages as they were transferred between
neighbor nodes. Some messages got transmitted over up to 172 meters,
with an average distance of 56 meters. Such rather long transmission
distances could of course be observed because the volunteers stayed
outdoors while roaming the campus.

The figure below shows a projection of all the message transfers that
were recorded during the field experiment (click on the image to see
it in full size). Each red segment depicts the transfer of a message
from a sender to a receiver.

As expected, most of the transmissions recorded in the log files
occurred over line-of-sight paths. Yet, the figure shows that some
transmissions also occurred through buildings. This can also be
observed when playing the video. Indeed, a thorough observation of
these transmissions through buildings shows that they actually
occurred over the lowest buildings of the campus, and through openings
(e.g., doors and windows) in higher buildings. In any case, observing
that small mobile devices such as smartphones operating in ad hoc mode
managed to exchange messages in such challenged conditions, and over
such distances, is an unexpected outcome of this field experiment.

The figure below shows the cumulated distribution function of the
distances covered as messages got transferred between neighbor
nodes. It can be observed that only a small amount of transmissions
were performed at short range (e.g., 12% below 20 meters), while a
significant amount of transmissions were performed at rather long
range (e.g., 40% above 60 meters).

The fact that many transmissions occurred over long distances is a
direct consequence of the way DoDWAN operates on mobile nodes. Indeed,
as soon as a radio contact is established between two nodes, they
start gossiping and try to exchange the messages stored in their
cache. Once this exchange phase is complete, the two neighbor nodes do
not have anything more to exchange, unless new messages are published
locally on either of them, or unless one of them manages to obtain new
messages from a third-party node.

This behavior is illustrated in the figure below, which shows what
happened during a contact that occurred between nodes dmis12
and dmis13 during the field experiment. This contact lasted
exactly 1'46" (from 15:29:22 to 15:31:08).

The purple line in the figure shows the evolution of the distance
between nodes dmis12 and dmis13 during this contact. It
can be observed that the contact was established as the two nodes were
almost 160 meters apart. This distance progressively decreased until
the nodes were only 2 meters apart, then it increased again until the
contact was broken. Such a V-shaped line depicts typically the
evolution of the distance between two mobile devices whose carriers
first walk toward each other, pass each other, and finally walk away.

Each green vertical impulse in the figure marks the transmission of
a message between nodes dmis12 and dmis13 (in any
direction). 38 messages got exchanged during that contact, but 29 of
them actually got transferred during the first 14" of the contact, as
both nodes still had plenty of original messages in their local
cache. This is a perfect illustration of the standard behavior of
DoDWAN nodes, as described above, and it explains why a majority of
message transfers occurred over long distances during the field
experiment.

Message dissemination

The volunteers published 249 messages during this experiment, and
2234 receive events were recorded in the log files. Since each message
published on a smartphone could be received by at most 9 other
smartphones, the delivery ratio is 2234/(249*9)=99.6%. Indeed, only
two messages were not received by all possible receivers. These two
messages were both published by node dmis01 at 15:42:26 and
15:43:07 respectively, that is, only a few minutes before the
experiment's deadline, and as the smartphones were progressively being
turned off.

Message delivery delays

As shown in Table I, the average delivery delay observed in the log
files is 2'30", and the maximum delivery delay is 18'16". The figure
below shows the cumulated distribution function of the average delays
observed for all the message deliveries recorded in the log files. It
can be observed that 50% of the deliveries occurred less than 2'20"
after a message was published, and 90% of the deliveries occurred in
less than 4'00".

Illustration of the dissemination of one message

The short video below shows the propagation of a single message,
that was published on node dmis09 at 15:40:33 during the field
experiment. This message then propagated from node to node, and it
reached the latest node at 15:41:24, that is, 51 seconds later.

In this video, a node is marked with a blue circle as soon as it
becomes a carrier of the message originally published by
node dmis09. The video starts 5 seconds before the message is
published on node dmis09, and it terminates 5 seconds after the
last delivery of the message.

As shown in this video, as soon as the message got published on
node dmis09 it started disseminating, and reached 6 other nodes
in less than 2.5". Yet it still took 48.5" for the message to reach
the 3 remaining nodes.

While watching this video, one may wonder why the dissemination of
the message is not faster. One of the reasons is that establishing a
contact between two nodes is sometimes difficult, because of radio
interferences. Another reason is that the single message we are
following in this video is not the only message the nodes must
exchange. Two nodes engaged in a radio contact may have dozens of
other messages to exchange. This is why passing a message from one
node to the next sometimes takes a while.